From the CDR, modifications and improvements were made with regards to safety, weight, and
manufacturability. While the frame remained unchanged, the ramp and stabilizing outriggers underwent considerable redesigns. Our final design allows any user to easily load themselves onto the board and stay securely attached and balanced while performing an activity. An exploded view highlighting each subsystem along with the fully assembled system is shown in Figure 5.1. Detailed parts, Bill of Materials, and dimensioned drawings can be found in Appendix E.
Page | 22 Figure 5.1 Exploded view of overall design assembly
The material of the loading ramp, which was initially metal sheets, was replaced with High Density Polyethylene (HDPE) sheets. The middle section was also shortened as our ramp test (Appendix H) revealed that there was too much deflection. The pontoons are now manufactured by our team, as there were not any products on the market that met our specifications. Lastly, D-ring pins were chosen as a quick way to secure the stabilizing outrigger system.
5.1.1
Attachment Frame
Sitting above our paddleboard is the attachment frame, which serves as a method to secure the user’s wheelchair and outriggers onto the paddleboard. Much of the design has remained the same, consisting of three components as shown in Figure 5.2: the longitudinal rail, transverse box beam, and outrigger strut housing.
Page | 23 Figure 5.2 Exploded model of frame subsystem
The longitudinal rail is an 8’ x 2’’ x 3/8’’ extrusion, the transverse box beam is 32’’ x 1’’ x 2’’ with a 1/8’’ thickness, and the outrigger strut housing is a 1.8’’ box beam with a 3/16’’ thickness. All components will be 6061 T6 Aluminum, which we have found to be the best grade in terms of water and corrosion resistance. The components will be joined with a lap weld where the edges of each part meet (details of the manufacturing plan can be found in section 6). The rigid, rectangular frame design will minimize any possibility of the frame slipping over any edge of the paddleboard.
We decided on altering the outrigger strut housing design to a box beam for ease of manufacturing. Our initial design was composed of a round tubing welded into a three-faced box beam. The new design will eliminate an additional concern for failure at a weld as well improve ease of reproducibility. We also drilled a ¼’’ hole on each front and back face of the housing to fit a D-ring. The round poles extending to the pontoons will have a minimal clearance fit into the square strut housing to ensure that there is no “wobbling” of the poles. The design for the transverse box beam has remained unchanged as it would provide the most structural rigidity.
In addition to the frame are eight straps from Northwest River Supplies (NRS) responsible for securing the frame onto the paddleboard, shown in an exploded view in Figure 5.3. The straps will wrap around the underside of the paddleboard and over the top of the frame. One of the main concerns was that having eight straps on the underside of the board would produce significant fluid drag in the water. To test the feasibility of our design, we conducted a hydrodynamic test by loading eight straps onto a paddleboard and testing the time it takes to travel a fixed distance under a constant force. The straps ended up having a small but negligible effect on the dynamics of the paddleboard and we concluded that they would not be an issue. More details regarding the test can be found in Appendix F.
Page | 24 Figure 5.3 Exploded view of frame and paddleboard
With a combination of the NRS straps and the rigidity of the frame, the user and its wheelchair will be able to remain fixed on top of the paddleboard without concern of the frame slipping off.
5.1.2
Anchor Straps
On top of the transverse box beam sits the anchor straps that will be responsible for connecting the frame to the wheelchair. Since CDR, we have kept the location of the strap to the top of the transverse beam to eliminate potential wear from the edge of the beam.
Page | 25 To meet the requirement of accommodating a variety of wheelchair sizes and designs, a Velcro strap system is utilized to allow continuous adjustability of the loop length that attaches though the frame of a wheelchair. We were able to manufacture our own Velcro straps and are confident based on the quality of sewing that we produced a quality strap.
Figure 5.5 Velcro strap attachment to chair
As displayed in Figure 5.5, Velcro is sewn on to one side of the nylon webbing to allow a secure
temporary loop to be formed around a member of the user’s wheelchair. The Velcro is secured through a buckle that will connect to the nylon strap portion that is mounted to the frame. The Velcro also has a fair amount of surface area of contact to ensure the method of securing the straps in place will not fail under proper use.
A tensioning buckle is included on each strap to allow for pre-tensioning. This eliminates the possibility of slack in the attachment straps and ensures a tight clamping force between the board and the wheelchair.
On the side of the strap that attaches to the aluminum frame, the webbing is captured between a fender washer and the rectangular aluminum tube. The fender washer helps evenly distribute the clamping force over a wider area of the strap. An exploded view of the sub assembly can be seen in Figure 5.4. Each of the four attachment points will have a 3/8’’ bolt, two 3/8’’ washers, a 3/8’’ fender washer, and a nut.
A main concern for the strap connection design was strap failure under a tensile load. We ran the strap configuration through a tensile tester and in each test case, the strength of the low-budget test webbing exceeded our predicted calculations for worst case scenario load. This test eliminated any concern for strap failure due to the strap and connection withstanding more force than the worst-case loading scenario. More details of the test and predicted load can be found in Appendix J.
An important consideration for this design is to minimize sharp points and edges on the tips of the bolts and ends of the transverse beams. To address any potential dangers to the users or instructors that interact with the paddleboard, a protective rubber adhesive is placed on protruding corners.
Page | 26
5.1.3
Stabilizing Outriggers
Our stabilization outriggers will consist of two pontoons mounted at either end of the aluminum struts that extend from the frame. These struts will pass completely though the pontoons, allowing for the adjustment of the pontoons along their length. The location of the pontoons will be fixed with D-ring pins.
To ensure adequate room for the end user’s paddle stroke and sufficient buoyancy, the pontoons will be 10’ x 8” x 8”.As shown in Figure 5.6, they will follow a hydrodynamic curve reminiscent of a sculling boat.
Figure 5.6 CAD model of outrigger pontoon
After discussing manufacturing options with local fiberglass expert and former Cal Poly shop director George Leone, we selected a polyurethane (PU) foam and UV-cure polyester resign design. The pontoons consist of a 1 ½” wall of PU foam and three layers of glass cloth saturated with resin and topped in a wax ‘hot-coat’ layer to provide a complete cure.
From George Leone’s advice and the process documentation of the 2013 Cal Poly senior project ‘Human Powered Hydrofoil’ we concluded that it would be feasible to assemble a layered foam design using the more readily available 1” sheets of PU foam instead of a solid 10’ block. We cut individual layers out of PU foam and adhered them together with a water-activated foaming glue – Gorilla Glue – before sanding to shape and glassing.
The aluminum strut housing inserts are placed into holes drilled through the bow and stern of the pontoon and epoxied in place. The location of the holes was determined by conducting a simple test of placing the board and pontoons into the water and seeing where they floated relative to each other. Because the holes sit so high on the pontoons, additional reinforcement was needed ensure that the pontoon would not fail at a given load. After the inserts were secured, we used carbon fiber and several additional layers of fiberglass to reinforce that area. The tubes are offset from the outer face of the pontoon on one side and have a through-hole perpendicular to the tube to allow for the wire snap pin to lock to the struts.
Page | 27 To validate our process and develop a familiarity with fiberglass George Leone invited us to his
workshop in Atascadero and walked us through the process of developing four test samples of fiberglass on a scrap surfboard core. Additionally, the results of our tipability test in Appendix G show that the outrigger system holds up extremely well to large loads and cannot flip the paddleboard.
5.1.4
Loading Ramp
After considerations of our previous ramp, we decided to switch to an HDPE double ramp which cuts down on weight and provides more strength (calculations of HDPE strength can be found in Appendix K). We modified the lengths of the middle section of the ramp to reduce deflection and added corners to both sides of each ramp to ensure a safe loading for the user.
Figure 5.7 CAD Model of Ramp
The loading ramp, seen in Figure 5-g, is constructed of two parallel longitudinal rails, which are hinged and broken into three sections. The first sections, onto which the user will initially begin loading, are 8” x 36” x 1”. The second middle sections, going between the loading surface and the board, are also 8” x 36” x 1”. This was the section we shortened to reduce deflection. The third sections, which go over the struts on the board, are 12” x 25” x 1”. Three horizontal crossmembers connect the parallel rails. The crossmember sitting at the very beginning of the ramp is 3” x 48” x 1”. Another crossmember sitting at the hinges between the first and second rails is 3” x 30” x 1”. The third crossmember sits between the
Page | 28 second and third rail sections measures 10” x 30” x 1”. This crossmember is supported by two support blocks which raise it off the board surface. These blocks are angled and cut from sections of rubber blocks. One block rest under each side of the crossmember. A pair of smaller angled support blocks sits under the third ramp sections. The support blocks are connected to the boards with #8 x 2” construction screws.
Steel hinges are used to connect the ramp sections. Two 3-1/2” wide hinges are placed side by side on a single rail section at each connection. The hinges are screwed into the ramp with #8 x 1-1/4”
construction screws, and eight hinges are used in total. Layers have been removed from the boards to allow the hinges to sit flush with the rest of the ramp surface. The ramp will be coated with a sealant to protect against water damage and a layer on the upper faces to provide additional traction.
The purpose of the three-section hinged ramp is to provide versatility and portability. The middle section can pivot to slope upward and downward, allowing for loading from a shore or from a dock. The sections at the beginning and end will sit firmly on the initial loading surface and the board itself, respectively. These sections provide stability during loading and a smooth transition on and off the ramp. The crossmember at the beginning of the ramp extends wide to either side of the parallel rails. A person who is assisting in loading the user onto the board places their feet on either end of this
crossmember. The weight of them standing on the member helps keep the ramp in place during loading. Once on the ramp, the user must be able to get over the protruding parts of the board frame. For this reason, the last sections of the ramp are supported by blocks which sit on either of the housing tube for the outrigger struts. The support blocks keep the ramp raised above the frame so the user can roll easily over the housing tubes. The ramp then angles the user smoothly down onto the board surface. The L- shaped brackets running along the edge of the ramp function as a wall that prevents the user from rolling sideways off the ramp during loading.
Our ramp design was tested by building a full-scale prototype out of plywood and loading a person in a wheelchair onto the paddleboard. The test, which is discussed in greater detail in Appendix G, proved that the design could effectively and comfortably allow a user to load themselves onto the paddleboard with minimal assistance. The entire process was simple and expedient.
Our loading ramp provides a versatile way in which users can load themselves onto the board. Our design is lighter, more compact, and more rigid than previous revisions.